Your computer uses DDR2 RAM.
If you want, you can download a program called CPU-Z here
. And then when you open the program look at the memory tab, and it will tell you the type and speed of RAM your computer uses.
I think an extra gig will do you good. Most of the time doubling your RAM up to 2GB will see a nice improvement. Especially with how smooth things run. That's if your system is really free of virus, etc.
You can also look up various Vista tweaks that will squeeze a little more performance out of your computer. As well as making sure everything is up to date. OS, drivers, etc.
In terms of investing in other hardware, that would depend on what you use your computer for. If it's just for surfing the web, music, movies, the general, then you don't really need to look further than a memory upgrade.